1. Field of the Invention
The present invention relates to a method and an apparatus for data transmission in an Orthogonal Frequency Division Multiplexing (OFDM) system. More particularly, the present invention relates to a method and an apparatus for initial frequency synchronization for data transmission by using OFDM symbols.
2. Description of the Related Art
The OFDM scheme is a fourth generation (4G) modulation scheme which is expected to be adopted as a standard for digital televisions in European countries, Japan and Australia. The OFDM scheme was initially recommended for a Local Area Network (LAN) technology and is now being developed to provide mobility to an OFDM-based wireless LAN technology for a cellular system for wireless Internet service.
The band spread technology of the OFDM scheme distributes data to a plurality of sub-carriers at the exact same frequency interval. This frequency interval provides “orthogonality” within a technology preventing a demodulator from referring to frequencies other than its own frequency. Further, the OFDM scheme is a kind of multi-carrier modulation scheme and shows an excellent performance in a multi-path mobile reception environment. Therefore, the OFDM scheme is now attracting attention as a modulation scheme that is well suited for ground wave digital televisions and digital voice broadcasting. Although the OFDM scheme has been researched and developed mainly in the field of communication, it is now being actively researched and developed in the field of broadcasting, especially in the case of broadcasting after the OFDM scheme is employed as a modulation scheme of a Digital Audio Broadcast (DAB) system proposed by the European Broadcasting Union (EBU).
FIG. 1 is a block diagram illustrating the structure of a transmitter and a receiver in a physical layer of a typical OFDM system.
Referring to FIG. 1, an input bit stream to be transmitted is transferred through an encoder 111 to a serial/parallel converter 112. Then, the serial/parallel converter 112 collects N number of symbols and transfers the N symbols to an Inverse Fast Fourier Transform (IFFT) converter 113 which converts the symbols from symbols of the frequency domain into time domain symbols. Thereafter, a parallel/serial converter 114 converts the time domain symbols into serial symbols. In the above process, the N collected symbols are referred to as ‘OFDM’ symbols. Then, a Cyclic Prefix (CP) inserter 115 adds a CP to each of the serial time domain symbols obtained by the parallel/serial converter 114 in order to remove the influence of the multi-path channels. Then, the CP-added symbols in a digital domain are converted to an analog signal by a digital/analog converter 116, and the converted analog signal is then transmitted through a channel 120 to a receiver side.
When the transmitted signal is received by the receiver 130 through the channel 120, an analog/digital converter 131 converts the received analog signal into a digital signal, and a CP remover 132 removes CP from the OFDM symbol contaminated due to the multi-path. The CP-removed signal is converted into a frequency domain signal by a Fast Fourier Transform (FFT) converter 134 after passing through a serial/parallel converter 133. The converted frequency domain signal passes through an equalizer 135 for eliminating channel interference, a parallel/serial converter 136 and a decoder 137, and is then output as an output bit stream at the receiver terminal.
FIG. 2 is a graph showing data symbols transmitted in a typical OFDM system, which are illustrated according to frequency and time.
In an OFDM system as described above with reference to FIG. 1, N number of data symbols within one OFDM symbol are transmitted by N number of sub-carriers. The N data symbols carried by the N sub-carriers constitute one OFDM symbol 201, and M number of OFDM symbols constitute one frame 202. The start symbol of the frame 202 usually includes a pilot symbol for frequency synchronization and channel estimation, by which a preamble, control information and so forth, is transmitted.
The OFDM system has an excellent performance for a mobile reception environment and a good frequency band use efficiency. However, in the OFDM system, the sub-carriers orthogonal to each other are compactly disposed with small intervals. Therefore, the OFDM system is relatively weak with regard to frequency offset in comparison with the single sub-carrier system.
Hereinafter, an example of the orthogonality between sub-carriers in the OFDM system will be described with reference to FIG. 3.
Referring to the graph in FIG. 3, three sub-carriers a shown. It is noted that data is transmitted by using frequency fn−1 301, frequency fn 302 and frequency fn+1 303 adjacent to each other. The data transmitted through each of the frequencies 301 to 303 has a sinusoidal waveform, and each of the first frequency signal 304, second frequency signal 305 and third frequency signal 306 is exactly located at the frequency of a corresponding sub-carrier. Therefore, the three signals give no interference to each other.
FIG. 4 is a graph showing interference between sub-carriers when there are frequency offsets in a typical OFDM system.
If each sub-carrier has a frequency offset of Δf 401 from an exact frequency of the sub-carrier, the receiver fails to catch the exact frequency location of the sub-carrier and instead takes a data sample at a location deviated Δf 401 from the exact location. Therefore, interference occurs between the three sub-carriers shown in FIG. 4, including the first sub-carrier signal 402, the second sub-carrier signal 403 and the third sub-carrier signal 404. For example, a signal sample 405 having a frequency offset of Δf 401 from the second sub-carrier signal 403 is subject to interference by the first sub-carrier signal 407 and the third sub-carrier signal 406 at a corresponding frequency location. As described above, the OFDM system has orthogonality between sub-carriers, which reduces the interval between the sub-carriers and causes the sub-carriers to be compactly arranged. Therefore, the OFDM system is largely influenced by interference due to the frequency offset.
According to a conventional initial frequency synchronization scheme in order to compensate for frequency offsets in the OFDM system as described above, the initial frequency synchronization is performed by using two pilot OFDM symbols. The conventional initial frequency synchronization includes two steps, that is, a first step of fine frequency synchronization (that is, compensation for frequency offsets within a band twice as wide as the sub-carrier band) and a second step of frequency ambiguity resolution for a part corresponding to a multiple of the band that is twice as wide as the sub-carrier band.
FIG. 5 illustrates an example of the format of pilot OFDM symbols according to the conventional initial frequency synchronization method in an OFDM system.
The first pilot OFDM symbol 501, which is a symbol for the fine frequency synchronization (hereinafter, referred to as “the first frequency synchronization”) in the first step for the frequency synchronization, has values other than zero for the sub-carriers in an even number order and ‘0’ for the sub-carriers in an odd number order. The first pilot OFDM symbol 501 is identical to the repetition of pilot symbols each having a half symbol length in the time domain. The process for the first frequency synchronization corresponds to a process of obtaining the decimal part of the frequency offset.
The second pilot OFDM symbol 502 is a symbol for the frequency ambiguity resolution in the second step for the frequency synchronization, which will be referred to as the second frequency synchronization process. The second pilot OFDM symbol 502 has values for all sub-carriers. The second frequency synchronization process corresponds to a process of obtaining the integer part of the frequency offset.
In other words, the frequency offset includes a decimal part expressed as being smaller than twice the sub-carrier band, and an integer part expressed as being a multiple of twice the sub-carrier band, which can be expressed by equation (1) below.Δf=φ/(πT)+2g/T  (1)
In equation (1), Δf denotes the entire frequency offset, φ denotes the decimal part of the frequency offset and T denotes the symbol length. Further, g denotes the integer part of the frequency offset corresponding to an integer being a multiple of twice the sub-carrier band.
If one data symbol is expressed by using an N point Fast Fourier Transform (FFT), a received symbol signal w(t) having a frequency offset can be expressed by equation (2) below.
                                          w            ⁡                          (              t              )                                =                                                    ∑                                  -                  N                                            N                        ⁢                                          H                k                            ⁢                              C                k                            ⁢                              exp                ⁡                                  (                                                            j2π                      ⁡                                              (                                                                              f                            k                                                    +                                                      Δ                            ⁢                                                                                                                  ⁢                            f                                                                          )                                                              ⁢                    t                                    )                                                                    ⁢                                  ⁢                              w            ⁡                          (              t              )                                =                                    exp              ⁡                              (                                  j2πΔ                  ⁢                                                                          ⁢                  f                  ⁢                                                                          ⁢                  t                                )                                      ⁢                                                            ∑                                      -                    N                                                  N                            ⁢                                                H                  k                                ⁢                                  C                  k                                ⁢                                  exp                  ⁡                                      (                                          j2π                      ⁢                                                                                          ⁢                                              f                        k                                            ⁢                      t                                        )                                                                                                          (        2        )            
If one received data symbol located at the first half OFDM symbol of the first pilot symbol is set as w(t0), and another received data symbol located at the second half OFDM symbol corresponding to the same location of the first half OFDM symbol is set as w(t0+T/2), a relation as expressed by equation (3) below is established.
                                                        w              *                        ⁡                          (                              t                0                            )                                ⁢                      w            ⁡                          (                                                t                  0                                +                                  T                  /                  2                                            )                                      =                              exp            ⁡                          (                              jπΔ                ⁢                                                                  ⁢                fT                            )                                ⁢                                    ∑                              k                =                                  -                  N                                            N                        ⁢                                          ∑                                  l                  =                                      -                    N                                                  N                            ⁢                                                H                  k                  *                                ⁢                                  C                  k                  *                                ⁢                                  H                  l                                ⁢                                  C                  l                                ⁢                                  exp                  ⁡                                      (                                                                                            j2π                          ⁡                                                      (                                                                                          f                                k                                                            -                                                              f                                l                                                                                      )                                                                          ⁢                                                  t                          0                                                                    +                                              jπ                        ⁢                                                                                                  ⁢                                                  f                          1                                                ⁢                        T                                                              )                                                                                                          (        3        )            
If the phase of the correlation value of the repeated half OFDM symbol length from equation (3), a relation defined by equation (4) below can be established between the decimal part φ of the frequency offset and the entire frequency offset Δf.φ=πΔfT  (4)
That is, it is possible to estimate the decimal part of the frequency offset through phase estimation by taking the correlation coefficient of the repeated part of the first pilot symbol.
The conventional frequency offset estimation method teaches the use of the function as defined by equation (5) below in order to enhance the precision in the estimation.
                              P          ⁡                      (            d            )                          =                              ∑                          m              =              0                                      L              -              1                                ⁢                                    w                              d                +                m                            *                        ⁢                          w                              d                +                m                +                L                                                                        (        5        )            
If it is possible to guarantee that the absolute value of the initial frequency offset is within a range smaller than the sub-carrier band, the entire frequency offset can be estimated as equation (6) below.Δ{circumflex over (f)}={circumflex over (φ)}/(πT)  (6)
However, it is actually not always possible to guarantee that the absolute value of the initial frequency offset is within a range smaller than the sub-carrier band, and there exists an ambiguity corresponding to a multiple of twice the sub-carrier band.
Therefore, in order to finally determine the frequency offset, it is necessary to resolve the ambiguity of the integer part of the frequency offset corresponding to the integer g in equation (1) in advance, for which the second pilot symbol is used. First, an estimation for the part corresponding to the decimal part of the frequency offset in the first frequency synchronization process is performed. Then, only the part corresponding to 2g/T remains in the frequency offset.
The frequency conversion values of the first and second pilot symbols are set as X1,k and X2,k, respectively, and the second pilot symbol is determined such that the differentially modulated values of the frequency conversion value of the first pilot symbol and the frequency conversion value of the even number-th sub-carriers of the second pilot symbol have a particular pattern. Further, according the conventional method, in order to determine g which is the integer part of the frequency offset, correlation coefficients of a predetermined pattern and differences between the first and second pilot symbols for possible g values are obtained. Then, a g value having the largest correlation value from among the obtained correlation values is determined as the final value. Through the above process, the frequency offset is estimated.
FIG. 6 is a block diagram illustrating a structure of a transmitter and a receiver for initial frequency synchronization in a physical layer of a conventional OFDM system.
In the transmitter 610 of FIG. 6, for transmission through the first and second OFDM symbols of each frame as described above, pilot bits pass through the serial/parallel converter 612, are converted into symbols of the time domain, and are then converted into a serial pilot bits. The Cyclic Prefix (CP) inserter 615 inserts CPs to the converted pilot bits, which are then converted from the digital signal to an analog signal by the digital/analog converter 616. Then, the converted analog signal is transmitted through the channel 620 to the receiver 630.
The signal received through the channel 620 is converted again from the analog signal to a digital signal by the analog/digital converter 631 and the converted digital signal is then transferred to the correlator 638. Then, in order to acquire the first frequency synchronization, the correlator 638 finds the repeated pattern of the first pilot OFDM symbol of the received signal and revises the decimal part of the frequency offset. After the decimal part of the frequency offset is revised, the CP remover 632 removes CPs from the received signal, the serial/parallel converter 633 converts the signal into a parallel signal, and then the FFT converter 634 converts the signal into a signal of the frequency domain. Then, in order to acquire the second frequency synchronization, the ambiguity resolution unit 640 checks the correlation value between the differential values of the first and second pilot symbols in the frequency domain and thereby solves the ambiguity of the frequency offset, that is, revises the integer part of the frequency offset. Then, the initial frequency synchronization is acquired as a result.
The conventional frequency synchronization method as described above is known as a method that is capable of obtaining an exact initial frequency offset. However, in the OFDM based wireless system, the conventional method uses two OFDM symbols within one frame for pilot transmission in order to revise the initial offset, thereby causing an excessively large overhead. In order to solve this problem, a method has been developed that is capable of acquiring the initial frequency offset while using only one pilot OFDM symbol for revising the initial frequency offset.
According to this method which uses only one pilot OFDM symbol, the second pilot OFDM symbol as shown in FIG. 5 is not transmitted, and only the first pilot OFDM symbol is transmitted, so that the symbol corresponding to the second pilot OFDM symbol can be used in transmitting data and thus can reduce the overhead. In the method for initial frequency synchronization by using only one pilot OFDM symbol, it is also necessary to perform both the first frequency synchronization step for finding the decimal part of the frequency offset, and the second frequency synchronization step for resolving the ambiguity of the multiple of the sub-carrier band by finding the integer part of the frequency offset. That is, the same process as that using equation (6) is also used in the method for initial frequency synchronization by using only one pilot OFDM symbol.
However, although the method using only one pilot OFDM symbol can reduce the overhead of the system in comparison with the method using two pilot OFDM symbols, the method using only one pilot OFDM symbol is based on an assumption that the channel does not change for a predetermined number of sub-carriers in resolving the ambiguity for determining the integer part of the frequency offset. Therefore, the method using only one pilot OFDM symbol has a degraded performance in acquiring the initial frequency synchronization for a channel environment having a selectivity in the frequency domain.
Accordingly, a need exists for a system and method that is capable of substantially guaranteeing an improved performance for initial frequency synchronization while reducing the overhead of the system.